Definition: A meta description is an HTML attribute that offers a brief explanation of the information on a webpage to be shown on a Search Engine Results Page (SERP). While they don't affect rankings, descriptions can improve clickthrough rates by distinguishing the webpage from similar results.
On a search engine results page, meta descriptions appear just below the page title. As an example, consider an individual who uses a search engine to find shower heads. The results provide a variety of links pertaining to shower heads and underneath each is a short summary of the link's contents and/or its keywords.
Within source code, a meta description is included within the <head> element and looks like this:
<meta name="description" content="Buy customized wrist watches from the web's most dependable source of fashion accessories."/>
SERPs will only show the first 155-160 words.
Page titles and URLs are a searcher's primary means of deciding which result to click on. A description should complement the title and give user's a reason to find out more. Effective descriptions:
Though writing a meta description is a best practice, it's important to know that search engines don't always show them in the SERPs. Search engines want users to feel compelled to click on their results, and they algorithmically determine whether to use a supplied description or other on-page content.
When the description is not used, content from the page is included in its place. The scraped description often includes keywords relevant to the user's query. This makes auto-generated descriptions potentially more relevant than the pre-determined, one-size-fits-all description. Pages geared towards long-tail search terms can actually benefit from scraped descriptions, which can greatly reduce the manual workload for websites with high product counts.